This invention is related to the art of making alkaline-earth-metal-aluminum-oxide and alkaline-earth-metal-lanthamide-aluminum-oxide materials.
Sol-gel processes allow for the low temperature preparation of aluminum oxide materials of high purity and controlled microstructure. These materials can be in such forms as gels, aerogels and xerogels. Sol-gel processes have been found useful in preparing supported and unsupported inorganic membranes for chemical sensor and separation applications, for automobile catalyst washcoats, and for optical coatings for automotive glass. Additionally, these processes have been found useful to prepare monoliths, fibers and monosized fibers from sols.
In some applications, it is desirable to incorporate at least one other metal atom in the aluminum oxide material. For example, it has been found desirable when employing the aluminum oxide material as a catalyst support to include atoms of lanthanum, cerium, calcium, strontium, barium, or all, in the aluminum oxide (alumina) matrix. Incorporating any of these metal atoms in the alumina matrix tends to prevent structural changes that occur in unstabilized .gamma.-alumina at high temperatures. In addition these metals are desirably included to provide oxygen storage in the alumina for catalytic applications involving oxidation.
When using sol-gel techniques to make the alumina material, these other metal atoms are added by co-hydrolyzing one or more metal alkoxides with aluminum alkoxide. As is well known in the art, such alkoxides when combined in water hydrolyze resulting in a mixture of hydroxides. Additionally, alkaline-earth-metals form insoluble alkoxides which are hard to purify. Thus, a mixture of alkaline-earth-metal alkoxide with aluminum alkoxide does not constitute a suitable precursor. For example, a mixture of aluminum alkoxide and lanthanum alkoxide on hydrolysis forms a mixture of aluminum hydroxide and lanthanum hydroxide. Some amount of mixed hydroxide is also formed. Undesirably the final product of such a mixture comprises a non-uniform two-phase distribution of metal oxide in an aluminum oxide matrix.